1. Field of the Invention
The present invention relates to a patch antenna for a radio communications apparatus capable of dual band operation. In the present specification, the term dual band antenna relates to an antenna which functions satisfactorily in two (or more) separate frequency bands but not in the unused spectrum between the bands.
2. Description of the Related Art
A patch antenna as known in the art comprises a substantially planar conductor, often rectangular or circular in shape. Such an antenna is fed by applying a voltage difference between a point on the antenna and a point on a ground conductor. The ground conductor is often planar and substantially parallel to the antenna, such a combination often being called a Planar Inverted-F Antenna (PIFA). When used in a cordless or cellular telephone handset, the ground conductor is generally provided by the handset body. The resonant frequency of a patch antenna can be modified by varying the location of the feed points and by the addition of extra short circuits between the conductors.
There are several advantages to the use of patch antennas in cordless or cellular telephone handsets, in particular a compact shape and good radiation patterns. However, the bandwidth of a patch antenna is limited and there is a direct relationship between the bandwidth of the antenna and the volume that it occupies.
Cellular radio communication systems typically have a 10% fractional bandwidth, which requires a relatively large antenna volume. Many such systems are frequency division duplex in which two separate portions of the overall spectrum are used, one for transmission and the other for reception. In some cases there is a significant portion of unused spectrum between the transmit and receive bands. For example, for UMTS (Universal Mobile Telecommunication System) the uplink and downlink frequencies are 1900-2025 MHz and 2110-2170 MHz respectively (ignoring the satellite component). This represents a total fractional bandwidth of 13.3% centred at 2035 MHz, of which the uplink fractional bandwidth is 6.4% centred at 1962.5 MHz and the downlink fractional bandwidth is 2.8% centred at 2140 MHz. Hence, approximately 30% of the total bandwidth is unused. If an antenna having a dual resonance could be designed, the overall bandwidth requirement could therefore be reduced and a smaller antenna used.
One known solution, disclosed in U.S. Pat. No. 4,367,474 and U.S. Pat. No. 4,777,490, is the provision of a short circuit between the conductors whose position is changed by switching using diodes, thereby enabling the operating frequency of the antenna to be switched. However, diodes are non-linear devices and may therefore generate intermodulation products. Further, in systems such as UMTS it is required to have simultaneous transmission and reception, so such switching is not acceptable.
An object of the present invention is to provide a patch antenna having dual band operation without switching.
According to a first aspect of the present invention there is provided a dual band patch antenna for a radio communications apparatus, comprising a substantially planar patch conductor, wherein a resonant circuit is connected between a point on the patch conductor and a point on a ground conductor.
According to a second aspect of the present invention there is provided a radio communications apparatus including an antenna made in accordance with the present invention.
The present invention is based upon the recognition, not present in the prior art, that by connecting a resonant circuit between a point on the patch conductor and a point on the ground conductor, the behaviour of the patch antenna is modified to provide dual band operation without the need for switching. Such an arrangement has the advantage that it can be passive and enables simultaneous transmission and/or reception in both frequency bands.
A patch antenna made in accordance with the present invention is suitable for a wide range of applications, particularly where simultaneous dual band operation is required. Examples of such applications include UMTS and GSM (Global System for Mobile communications) cellular telephony handsets, and devices for use in a HIPERLAN/2 (High PErformance Radio Local Area Network type 2) wireless local area network.
An unexpected advantage of a patch antenna made in accordance with the present invention is that the combined bandwidth of the two (or more) resonances is significantly greater than the bandwidth of an unmodified patch antenna without a resonant circuit. This advantage greatly enhances its suitability for use in typical wireless applications.